Copper (Cu) is replacing aluminum as the material of choice for wiring of microelectronic devices, such as microprocessors and memories. However, the presence of copper in semiconductors such as silicon causes defects that can prevent the proper functioning of transistors formed in the semiconductor. Copper also increases the leakage of current through insulators, such as silicon dioxide, placed between the copper wires. Therefore use of copper wiring demands that efficient diffusion barriers surround the copper wires, to keep the copper confined to its proper locations.
Copper also has a tendency to move in the direction that electrons are flowing in a circuit. This electromigration process can lead to increased electrical resistance, or even an open circuit if a sufficiently large void forms within a copper interconnection. Most of this unwanted motion takes place along the surface of the copper. Therefore it is critical to maintaining long lifetimes that the copper interconnections be surrounded by materials that inhibit electromigration. Tantalum metal (Ta) serves this function on the bottom and sides of currently-used copper interconnections. The top of copper wiring (those parts that do not connect to an upper level by a via) are covered by SiC or Si3N4, although these materials are not as effective as the Ta in preserving the copper against electromigration. The SiC or Si3N4 also have the disadvantage that they have a higher dielectric constant than the rest of the insulator, so they increase the capacitance of the circuits and decrease the speed with which signals can be transmitted through the wiring.
Improved lifetime against electromigration has been achieved by electroless deposition of cobalt-tungsten alloys containing phosphorous (CoWP) or boron (CoWB) selectively on top of the copper wires. This selective process is supposed to avoid any deposition of these electrically conductive alloys on the surface of the insulator. Thus it should lead to a self-aligned conductive diffusion barrier on top of all surfaces of the copper that were exposed by the CMP step. However, breakdown of the selectivity causes some electrical shorts over the insulators between Cu wires, making this process unreliable for mass production. Another disadvantage of this self-aligned process is that the alloy barrier remains on the parts of the Cu that will be later contacted by Cu-filled vias. In these areas, the CoWP or CoWB alloy becomes part of the electrical circuits, and increases their electrical resistance over the lower value they would have had without the alloy layer between the via and the Cu in the layer below.
Direct, low-resistance contact between copper vias and the copper below has been demonstrated by the use of sputtered Cu—Mn alloy seed layers followed by electroplating and then thermal annealing to form self-aligned MnSixOy diffusion barrier layers at the interface between the Cu—Mn alloy and the insulator. The thermal anneal is supposed to remove Mn from the Cu—Mn layer so that the remaining purer Cu has low resistance. The Mn diffuses either to form the MnSixOy layer on the insulator or it diffuses to the top, free surface of the Cu where it forms a MnOx layer (probably x=2) by reaction with oxygen in the anneal atmosphere. This MnOx layer is then removed along with the rest of the excess Cu during CMP. A disadvantage of this process is that Mn impurity is present in the Cu during the anneal that is supposed to increase the grain size and thereby decrease the electrical resistance of the Cu. The presence of the Mn impurity during the anneal can restrict the grain growth and thereby increase the final resistance of the Cu over what it would have been without the presence of the Mn impurity. Another disadvantage of this process is that some Mn impurity may remain in the Cu even after the anneal, thereby increasing its electrical resistance over that of pure Cu.
It has also been proposed to diffuse Mn to the upper surface of a copper interconnect in an oxygen-containing atmosphere in order to form an MnOx layer that could act as a diffusion barrier. However, such a MnOx layer has very weak adhesion to copper, and therefore the electromigration lifetime of such a structure is undesirably short.
It has further been proposed to use CVD or ALD with a manganese-containing metal-organic precursor and an oxygen-containing gas to form a manganese oxide barrier layer for copper. However, such a manganese oxide barrier layer forms on the surface of the insulator, rather than diffusing into the insulator. Thus the MnO barrier takes up space that otherwise could be occupied by conductive copper metal, and thus the resistance of a copper wire is undesirably increased. Also, the adhesion of copper to such a manganese oxide barrier may be less than desired for mechanical stability and long lifetime against electromigration.